In a traditional driving circuit for LED (Light Emitting Diode), after the supplied power has passed through a bridge rectifier and has been converted into a DC power, generally, it still requires extra processes through some electric elements including inductors and electrolytic capacitors, before driving any LEDs. However, the life of an electrolytic capacitor is relatively short, which has severely restricted the whole life of a driving circuit for LED, also, the conversion efficiency is usually as low as 80%. Thus, it has seriously affected the LED lighting effects.
In order to avoid the use of inductors and capacitors, designers have made some improvements on the driving circuit for LED, shown as FIG. 1, which is a circuit diagram in the patent of invention with a publication No. CN 102196627, wherein, three MOS transistors, 88, 87, 86, are applied to short-circuit or open-circuit the LED short string 62, LED short string 61, or LED short string 60. Take the MOS transistor 87 as an example, when it short-circuits the LED short string 61, the power voltage between L3 and L4 is close to zero; when it open-circuits the LED short string 61, the power voltage between L3 and L4 is 16N×3.0V (wherein, 3.0V is the power voltage between the two ends of a single LED, when it is on). Suppose the supplied power voltage is 220V, then, when all LEDs are lit-on, the voltage drop over the LEDs should be close to the highest voltage output from the bridge rectifier, which is, 220×1.414=311V, thus, the total number of LEDs should be around 100, and N should be at least 2. Therefore, when the MOS transistor 87 open-circuits the LED short string 61, the power voltage between L3 and L4 is 16N×3.0V>16×2×3.0V=96V. If all LEDs are lit-on, the power voltage of L3 to the ground should be 15×3.0V=45.0V
From the analysis above, it can be seen that, the lowest withstand voltage from the source of the MOS transistor 87 (connecting to L3) to the ground is 45V, the lowest withstand voltage from the drain of the MOS transistor 87 (connecting to L4) to the ground is 45+96=141V, while the lowest withstand voltage between the drain and the source of a standard MOS transistor 87 is 96V. Also, the driving circuit for the gate of the MOS transistor 87 (connecting to NG4) is also very complicated. In current arts for semi-conductor integrations, such a MOS transistor is impossible to be integrated. Therefore, the circuit configuration of the driving circuit is particularly complex, the requirements of the withstand voltages for each element are relatively high, and it is difficult to be integrated into one chip, therefore, it does not satisfy the increasing miniaturization requirements for electronic products. Shown as FIG. 2, which is a circuit diagram in the patent of invention with a publication No. US 2011/0084619A1, wherein, according to the claim 2, it can be seen that, through an AC (alternating current) synchronous circuit, different LED light strings are selected to be turned on at different time points, and the currents in the LED lamp strings are adjusted according to time. When driving LEDs, shutting 12B off and 12C on at a certain time point, the number of LEDs gets increased, and the current passing through those LEDs also gets increased. However, at the instant time point of turning off 12B, the voltage over the drain 122 in 12A will be very high, which is around the total number of LED lamps in both light-strings 11A and 11B times 3.0V, and, usually, the current in LEDs is 30 mA or higher. Suppose the total number of lamps in both 11A and 11B is 30, then, the power consumption of the chip will be 30×3.0V×30 mA=2700 mW, which means, the efficiency of a commonly used 7 W LED lighting system is less than 80%. Also, since the power consumption in the chip is 2.7 W, it will require a higher requirement for chip packaging.
And the light flux of an LED driven by a conventional AC driving apparatus will get higher following the rising of the supplied power voltages, which means a relatively large variation in light flux, also, in a conventional AC driving mode for LED, when the voltage is low, only a part of the LEDs in the LED string will get lit-on, and when the voltage is high, all LEDs will get lit-on, thus, during a sinusoidal voltage cycle, part of the LEDs are almost always in an off status, while other part of LEDs get lit-on during only ¼ cycle or even less, therefore, the attenuations of all LEDs are different, resulting in a worse lighting effects after a long time.
Therefore, the prior art needs to be improved and developed.